1. Field of Application
The present invention relates to a wireless communication apparatus in which the impedance of a receiving antenna is matched to the input impedance of a receiving circuit by an adjustable matching circuit.
2. Description of Related Art
A type of wireless communication apparatus is known, for example as described in Japanese patent publication No. 2007-290664 (designated in the following as reference document 1), having a receiving antenna and an antenna resonance circuit formed of inductors and a set of capacitors, with the capacitors controlled to function as a variable capacitor and the inductors serving to couple the antenna resonance circuit to the antenna. The antenna resonance circuit exhibits peak resonance at a specific frequency, so that a maximum level of signal power of a received signal is transferred from the antenna to a voltage detection circuit when the frequency of the received signal is equal to the resonance frequency of the antenna resonance circuit. Here “frequency of the received signal” signifies the carrier frequency of a modulated transmitted wireless signal. A capacitance adjustment circuit can adjust the capacitance in the antenna resonance circuit, to thereby adjust the resonance frequency. The apparatus also includes an oscillator for generating an adjustable-frequency test signal. The apparatus can be set in a tuning mode in which the test signal is applied to the antenna resonance circuit, with the test signal frequency being successively varied to detect the resonance frequency of the antenna resonance circuit, i.e., frequency at which peak output amplitude is obtained, and with the capacitance being adjusted to set that resonance frequency at a required reference value, that is, at a desired reception frequency. The adjustment is performed automatically by a control circuit, based on detected signal strength values that are obtained by the voltage detection circuit.
In the case of a wireless communication apparatus for high-frequency wireless operation, the resonance frequency of a receiving antenna itself (i.e., determined by the distributed inductance and capacitance parameters of the antenna) may be tuned to correspond to a desired reception signal frequency. Generally, a matching circuit is connected to the antenna and to the input of a receiving circuit, and serves to match the impedance of the antenna to the input impedance of the receiving circuit, and may also be adjustable for modifying the resonance frequency of the antenna. Ideally, the antenna resonance frequency (as modified by reactance values of the matching circuit) should correspond to a desired reception signal frequency, with the source impedance presented to the receiving circuit by the matching circuit corresponding to the input impedance of the receiving circuit, at that frequency.
However if any metallic objects are brought close to the antenna after the antenna resonance frequency has been set appropriately, the impedance of the antenna at that frequency will be altered. As a result, the antenna resonance frequency will be shifted from the desired value, and the impedance relationship between the antenna and the matching circuit will be altered. Reception sensitivity at the desired reception frequency will thereby be lowered. Hence, an automatic tuning function, for maintaining the antenna resonance frequency close to a specific reception frequency and maintaining impedance matching between the antenna and receiving circuit would be desirable.
It might be envisaged that the automatic tuning method of reference document 1 could be adapted for this purpose. However, that would have the disadvantage of requiring a variable-frequency oscillator for producing a test signal, and controlling the oscillator to generate successively different test signal frequencies when a tuning operation is being performed. Thus it would be necessary to provide such a variable-frequency oscillator as an additional component of a wireless communication apparatus, causing the circuit scale to be increased.
Furthermore, it might be envisaged that the automatic tuning function of reference document 1 could be simplified by using a fixed-frequency oscillator to produce a test signal at the desired reception frequency. A variable capacitor in the matching circuit could be successively varied, while the test signal is being applied to the matching circuit, until a peak value is reached by the output signal from the receiving circuit (voltage detection circuit). However when such an adjustment operation is commenced, there would be ambiguity as to whether the capacitance should be increased or decreased, to bring the matching circuit towards the optimum condition. Thus it might be necessary to successively adjust the capacitor over its entire range of variation, until a peak value of output signal from the receiving circuit is detected.